Activist investor Dan Loeb has taken a stake in the rental car company Hertz, which on Monday adopted a one-year shareholder rights plan after observing "unusual and substantial activity" in its shares, sources familiar with the situation told CNBC on Tuesday.
Loeb's hedge fund, Third Point, took a position in Hertz of less than 5 percent—not enough to require a regulatory filing, and not as large as his past stakes in Sony and Sotheby's.
Though sources told CNBC that Loeb doesn't plan to pursue an activist position, his presence could be a concern to Hertz leadership.
(Read more: Loeb: I don't want to see FedEx CEO replaced)
Hertz officials said Monday that the shareholder rights plan would go into effect only if a person or group were to acquire at least 10 percent of the company's stock.
Sources also told CNBC that former Carl Icahn protégé Keith Meister, of Corvex Capital, has added to his holdings in Hertz. Meister met with Hertz CEO Mark P. Frissora within the past month in what sources called a "constructive" encounter.
The amounts involved in both Third Point's and Corvex's investments suggest that Hertz could be concerned about a different investor or group taking a larger stake.
—Reported by CNBC's Scott Wapner and written by Jeff Morganteen. Follow Wapner on Twitter @ScottWapnerCNBC and Morganteen @jmorganteen. Reuters contributed to this story.